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Gigantic Rulers for Gigantic Distances

Science >

Distances in astronomy are, well, astronomical. It's not like we're measuring someone's shoe size here. We need rulers, gigantic rulers that can measure trillions of miles to get to even the next closest star. But even the biggest home improvement places can't supply that need.

So, when you read that Sirius is 8.6 light years away, or that the Andromeda galaxy is 2.6 million light years away, how do we really know?

There are a variety of mileage tricks up an astronomer's sleeve. Here we'll tackle just one, the one that helps us determine local distances. In future columns we'll take on the big scary long distance measurements.

You've been blessed with two eyes for a reason. Our eyes work in concert to give us a sense of depth. Each eye has a slightly different view of the world around us than the other. The brain can judge nearby distance pretty well using those views.

Put your thumb near your nose, but not in it! Look at it with just your right eye, then just your left. Notice each eye has a different thumby perspective.

Move your thumb slowly away, looking first with your right eye, then left, then right, then left. The thumb will appear to be moving back and forth less and less.

Imagine now you are Inspector Gadget and can extend your arm out another couple kilometers. Blink now and you'll notice no difference. Because our eyes are only about 6 cm from each other we have no sense of depth way out there. Our eyes are only good for a few hundred meters of depth perception at best.

To judge the distance of the thumb a couple kilometers away, you'd have to separate your eyes a couple meters from each other, which makes it tough to get through doorways.

But astronomers can separate their eyes by about 300 million kilometers and get a very good sense of depth out to amazing distances. How!?

Our eyes in astronomy are telescopes, and we can separate them so far because they move! Let's say we take an image of a nearby star in January. Then --- and here's the key --- six months later in July, when we are on the other side of the sun, we can take another image of the star to see how it has "moved" through the distant background stars.

Look at your thumb at arm's length. You right eye might represent Earth in January. Your left eye is Earth in July. (Your nose is the sun.) See how the starry thumb "moves" when you switch eyes? That is what nearby stars do from our point of view.

Nearby stars "move" a lot throughout the year. More distant star "move" less. This movement is called parallax. There is a simple equation that can change parallax to distance. But suffice it now to say that if one can measure the apparent angular movement of a star through the sky --- its parallax --- one can calculate distance.

Because of their enormous distances the parallaxes of stars are really, really tiny. Very few move more than an "arcsecond" annually. That's about the diameter of a Ping Pong ball 5 miles away!

That's why we need good scopes to do this hyperprecise measuring. One such telescope, the satellite Hipparcos, recently measured the parallaxes of over 120,000 nearby stars to thousandths of an arcsecond precision!

But our galaxy is over 100,000 light years across! Even our best equipment can measure star distances by parallax to only several hundred light years.

To measure farther we need something else…

Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.

Posted by Administrator at 2003.02.15 01:50 PM | Comments (0)

The Time Machine

Science >

Wouldn't it be nice to experience the past as it happened, to witness --- right now --- great events of eons long past?

Paleontologists could study dinosaurs in action. Anthropologists could observe ancient Egyptians building the pyramids. Philosophers could listen to Socrates in his own words.

Well, astronomers have been given this special gift; in fact we all have. It seems counterintuitive, but we can all look up into the heavens right now and watch the past!

It all boils down to the speed of light. Light does have a speed limit, albeit an incredibly fast one. Traveling over 186,000 miles in a second, light can cover a lot of ground in a little time.

But our vast universe takes up considerably more than a lot of ground.

Even something seemingly close like our moon is still 237,000 miles away and light needs time to get here; it isn't an instantaneous thing. It takes light more than a second to travel from the moon to your eyes.

And therein lies the point. If light takes over a second to get here, then we are seeing the moon as it was over a second ago, not as it is now. We are presently seeing the past. Got that?

The sun, about 93 million miles away, is finally appearing to us now as it actually was over 8 minutes ago. Big bright Jupiter is nearly 50 light minutes away; we see it as it was 50 minutes ago. Saturn's image is hitting our eyes about 70 minutes after it left the ringed planet. But we really get historic when we go out to the stars.

Brilliant Sirius, below Orion and 8.6 light years away, is appearing now the way it was when Mr Clinton was in the middle of his first term. More distant Betelgeuse, the orangish star on Orion's shoulder, just above Sirius, looks like it did when Elizabeth I was Queen of England back in 1580.

All this is more than just a pursuit in interesting astronomical trivia. It has real scientific merit. Astronomers can take advantage of the limited speed of light to see great distances back into the past and thus figure how the Whole Show has worked itself out.

For example, what they see is that we are in a relatively quiescent time in the history of the universe. As we look farther back, about 5-10 billion years, we can observe numerous blisteringly violent galaxies called quasars. Quasars, we believe, are galaxies of stars wrapped around a central supermassive blackhole.

These monsters are chewing up gas and stars and spitting out energy at unimaginably prodigious rates. That was a time when galaxies were closer to each other and there was widespread violent cannibalistic interaction between them. We aren't guessing at this, we know because we can see it happening!

We can also see that there was essentially only hydrogen and helium way back near the beginning; the periodic table was very incomplete.

The farther back we look the warmer it gets, giving support to the theory of a hot creation event billions of years ago.

We're now able to see far enough into the past to witness what appear to be the earliest infant galaxies coming together to form giants similar to the ones that rule the neighborhood in our time.

And the list goes on and on and on.

Go out tonight and witness history taking place before your eyes.

Until next time, clear skies!

Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.

Posted by Administrator at 2003.02. 1 01:52 PM | Comments (0)



Looking for something?	« January 2003 \| Main \| March 2003 » Gigantic Rulers for Gigantic Distances Science > Distances in astronomy are, well, astronomical. It's not like we're measuring someone's shoe size here. We need rulers, gigantic rulers that can measure trillions of miles to get to even the next closest star. But even the biggest home improvement places can't supply that need. So, when you read that Sirius is 8.6 light years away, or that the Andromeda galaxy is 2.6 million light years away, how do we really know? There are a variety of mileage tricks up an astronomer's sleeve. Here we'll tackle just one, the one that helps us determine local distances. In future columns we'll take on the big scary long distance measurements. You've been blessed with two eyes for a reason. Our eyes work in concert to give us a sense of depth. Each eye has a slightly different view of the world around us than the other. The brain can judge nearby distance pretty well using those views. Put your thumb near your nose, but not in it! Look at it with just your right eye, then just your left. Notice each eye has a different thumby perspective. Move your thumb slowly away, looking first with your right eye, then left, then right, then left. The thumb will appear to be moving back and forth less and less. Imagine now you are Inspector Gadget and can extend your arm out another couple kilometers. Blink now and you'll notice no difference. Because our eyes are only about 6 cm from each other we have no sense of depth way out there. Our eyes are only good for a few hundred meters of depth perception at best. To judge the distance of the thumb a couple kilometers away, you'd have to separate your eyes a couple meters from each other, which makes it tough to get through doorways. But astronomers can separate their eyes by about 300 million kilometers and get a very good sense of depth out to amazing distances. How!? Our eyes in astronomy are telescopes, and we can separate them so far because they move! Let's say we take an image of a nearby star in January. Then --- and here's the key --- six months later in July, when we are on the other side of the sun, we can take another image of the star to see how it has "moved" through the distant background stars. Look at your thumb at arm's length. You right eye might represent Earth in January. Your left eye is Earth in July. (Your nose is the sun.) See how the starry thumb "moves" when you switch eyes? That is what nearby stars do from our point of view. Nearby stars "move" a lot throughout the year. More distant star "move" less. This movement is called parallax. There is a simple equation that can change parallax to distance. But suffice it now to say that if one can measure the apparent angular movement of a star through the sky --- its parallax --- one can calculate distance. Because of their enormous distances the parallaxes of stars are really, really tiny. Very few move more than an "arcsecond" annually. That's about the diameter of a Ping Pong ball 5 miles away! That's why we need good scopes to do this hyperprecise measuring. One such telescope, the satellite Hipparcos, recently measured the parallaxes of over 120,000 nearby stars to thousandths of an arcsecond precision! But our galaxy is over 100,000 light years across! Even our best equipment can measure star distances by parallax to only several hundred light years. To measure farther we need something else… Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com. Posted by Administrator at 2003.02.15 01:50 PM \| Comments (0) The Time Machine Science > Wouldn't it be nice to experience the past as it happened, to witness --- right now --- great events of eons long past? Paleontologists could study dinosaurs in action. Anthropologists could observe ancient Egyptians building the pyramids. Philosophers could listen to Socrates in his own words. Well, astronomers have been given this special gift; in fact we all have. It seems counterintuitive, but we can all look up into the heavens right now and watch the past! It all boils down to the speed of light. Light does have a speed limit, albeit an incredibly fast one. Traveling over 186,000 miles in a second, light can cover a lot of ground in a little time. But our vast universe takes up considerably more than a lot of ground. Even something seemingly close like our moon is still 237,000 miles away and light needs time to get here; it isn't an instantaneous thing. It takes light more than a second to travel from the moon to your eyes. And therein lies the point. If light takes over a second to get here, then we are seeing the moon as it was over a second ago, not as it is now. We are presently seeing the past. Got that? The sun, about 93 million miles away, is finally appearing to us now as it actually was over 8 minutes ago. Big bright Jupiter is nearly 50 light minutes away; we see it as it was 50 minutes ago. Saturn's image is hitting our eyes about 70 minutes after it left the ringed planet. But we really get historic when we go out to the stars. Brilliant Sirius, below Orion and 8.6 light years away, is appearing now the way it was when Mr Clinton was in the middle of his first term. More distant Betelgeuse, the orangish star on Orion's shoulder, just above Sirius, looks like it did when Elizabeth I was Queen of England back in 1580. All this is more than just a pursuit in interesting astronomical trivia. It has real scientific merit. Astronomers can take advantage of the limited speed of light to see great distances back into the past and thus figure how the Whole Show has worked itself out. For example, what they see is that we are in a relatively quiescent time in the history of the universe. As we look farther back, about 5-10 billion years, we can observe numerous blisteringly violent galaxies called quasars. Quasars, we believe, are galaxies of stars wrapped around a central supermassive blackhole. These monsters are chewing up gas and stars and spitting out energy at unimaginably prodigious rates. That was a time when galaxies were closer to each other and there was widespread violent cannibalistic interaction between them. We aren't guessing at this, we know because we can see it happening! We can also see that there was essentially only hydrogen and helium way back near the beginning; the periodic table was very incomplete. The farther back we look the warmer it gets, giving support to the theory of a hot creation event billions of years ago. We're now able to see far enough into the past to witness what appear to be the earliest infant galaxies coming together to form giants similar to the ones that rule the neighborhood in our time. And the list goes on and on and on. Go out tonight and witness history taking place before your eyes. Until next time, clear skies! Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com. Posted by Administrator at 2003.02. 1 01:52 PM \| Comments (0)
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